Large quantities of natural gas, comprising mostly methane, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make the use of this remote natural gas economically unattractive in most instances. To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reacted to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels that boil in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, dry reforming (also called CO2 reforming), or partial oxidation. Steam reforming is the major process used commercially for the conversion of methane to synthesis gas and proceeds by the strongly endothermic reaction shown in Equation 1.CH4+H2OCO+3H2  (1)
Dry reforming, also endothermic, proceeds according to the reaction of Equation 2.CH4+CO22CO+2H2  (2)
These two processes require the input of appreciable quantities of heat to initiate and maintain the reaction, and thus, require equipment to provide that heat. Although steam reforming has been widely practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
In contrast, the partial oxidation of methane and other hydrocarbons is exothermic, and under ideal conditions can proceed according to the stoichiometry of Equation 3 to yield a syngas mixture with an H2:CO ratio of 2:1.CH4+1/2O2→CO+2H2  (3)
This ratio mixture is in many instances more useful than the H2:CO ratio from steam reforming for such downstream operations as the conversion of the syngas to chemicals such as methanol and to fuels.
Another advantage of utilizing partial oxidation processes for syngas production is that oxidation reactions are typically much faster than reforming reactions, and therefore, allow the use of much smaller reactors.
In order to avoid extreme conditions, various catalyst systems have been employed to catalyze the partial oxidation of hydrocarbons such as methane. In a typical catalytic partial oxidation process, the hydrocarbon feed is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The resulting product is primarily CO and H2. The product may also contain unreacted methane as well as other compounds such as H2O, CO2, and C2H4 which are products of secondary reactions. It may be preferable to increase the methane conversion and remove or reduce these secondary products.
It has been suggested that in catalytic partial oxidation bed, that the partial oxidation occurs in the first millimeter of bed length. See Hickman and Schmidt, SYNTHESIS GAS FORMATION BY DIRECT OXIDATION OF METHANE OVER PT MONOLITHS, 138 J. Catalysis 267, 275 (1992). Thus, because it is thought that only a small portion of the catalyst bed is needed to catalyze the reaction, it should logically follow that catalyst beds longer than 1 mm would waste expensive catalyst and reactor space. Additionally, in some instances, longer catalyst beds are undesirable because it is difficult or impossible to maintain high gas hourly space velocities and short contact times.
Although the above described methods of syngas production demonstrate significant advancements, there is a continuing need for better processes for the catalytic partial oxidation of hydrocarbons which produce a higher conversion of reactant gases and higher selectivity of CO and H2 reaction products (i.e., fewer secondary reaction products) and which are capable of operating at superatmospheric pressures without creating an undesirably large pressure drop across the catalyst bed.